[Appearance]
The insulator section and electrode section is black and lustrous with wet oily deposits. [Results]
Poor starting and misfiring.[Cause]
Oil leaking due to piston ring, cylinder, or valve guide wear (occurs easily to new engines and engines that have just been overhauled), high oil content in air-fuel mixture.(2-stroke engines)

[Appearance]
The spark plug is wet with gasoline immediately after it is removed, but it soon dries off.[Results]
Poor starting and misfiring.[Cause]
The air-fuel ratio is too rich and is not igniting. (Among the ways this can happen is if the driver presses the accelerator over and over while starting the vehicle.) [Handling]
Remove all the spark plugs, crank the starter motor to bring fresh air into the cylinder and make the air-fuel ratio leaner.

[Appearance]
The center and ground electrodes are rounded and the gap has become too wide.[Results]
Poor starting and acceleration.[Cause]
Inadequate maintenance. (spark plug has exceeded its service life.)

[Appearance]
The gap is wider than appropriate.[Results]
Poor starting and acceleration.[Cause]
The gap is inappropriate; the wrong spark plug was selected.
[Handling]
Remove all the spark plugs, crank the starter motor to bring fresh air into the cylinder and make the air-fuel ratio leaner.

Case 2
· Misfiring only when running at high speed or when accelerating suddenly

[Appearance]
The insulator leg section has yellow or yellowish-brown burnt on deposits or is covered with a glossy surface.[Results]
Misfiring during rapid acceleration or under high load, but no problem in normal running.[Cause]
Use of gasoline with much lead.

[Appearance]
The center electrode or ground electrode is melted or scorched. There are spots on the insulator leg section and deposits of aluminum or other metal powder. [Results]
Power loss due to engine damage.[Cause]
Often this is due to overheating; pre-ignition is a phenomenon in which combustion occurs before ignition.
The plug heat range is too low, the injection timing is too advanced, etc.

[Appearance]
The electrode is bent and the insulator leg section is broken. Indentations are sometimes seen on the electrode. [Results]
Misfiring.[Cause]
The spark plug thread reach is too long for the engine head or there is some kind of foreign matter (a small bolt, nut, or the like) in the combustion chamber.

[Cause]
Overheating of the ground electrode and severe engine vibration.

[Appearance]
Brown deposits on the insulator directly above the housing [Results]
No impact on the spark plug performance [Cause]
This occurs due to electrical stress in the air near the insulator. (This is not a spark plug gas leak, for which it is sometimes mistaken.)